A semiconductor element such as a switching element or a diode is used in a circuit such as a switching power source or an inverter. These semiconductor elements are required to have high breakdown voltage and low ON resistance. In general, there exists a tradeoff based on a material of the semiconductor element between the breakdown voltage and the ON resistance.
The lowering of the ON resistance of a semiconductor element is believed to be approaching the limit of silicon, which is a primary material of semiconductor device elements. Thus, to further lower the ON resistance, it is necessary to use elements other than silicon. With the use of a nitride semiconductor such as GaN or AlGaN or a wide band gap semiconductor such as a silicon carbide (SiC) as a material of a switching element, the tradeoff relationship can be improved, thus enabling a lowering of the ON resistance of a semiconductor device.
A High Electron Mobility Transistor (HEMT) which uses the AlGaN/GaN heterojunction structure is a nitride semiconductor with a low ON resistance, for example. The HEMT achieves the low ON resistance through the high mobility of electrons at a heterojunction interface channel and a high electron concentration generated by polarization.
Since electrons are generated by polarization in the HEMT a high concentration of electrons exist below a gate electrode. Accordingly, a HEMT is usually a normally-ON element (that is, the device channel is conductive unless a voltage is applied to the gate electrode) and the threshold voltage of the HEMT is generally negative. However, in view of a safe operation, there has been a demand for a normally-OFF element where a threshold voltage is positive. Further, it is desirable to increase a gate-on threshold voltage for increasing an ON current in the normally-OFF element.